Lime, primarily CaO, and dolo-lime, primarily CaO.MgO, are added to steel making vessels (electric arc furnaces, basic oxygen furnace (BOF) and Q-BOP, etc.) to form a complex melt of oxides, ordinarily called "slag," for the purpose of absorbing undesired impurities from the hot metal bath of molten iron. Slag in the steel making process is a mixture of CaO, Al.sub.2 O.sub.3, SiO.sub.2, MgO, and other mineral substances in small amounts including sulfur, and phosphorous, The silica, SiO.sub.2, sulfur, and phosphorous are the major impurities of the hot metal bath that the slag is intended to remove, or absorb.
The steel making operation entails melting scrap metal, and/or using molten iron and defining the melt to the desired chemistry to make steel products. The materials added to the steel making vessel are often termed "the charge", and adding materials to the vessel is called "charging". Lime and dolo-lime (and in some melt shops, bauxite) are part of the charge. As the melt is formed, the heat is melted in, or "formed up", the liquid metal bath is covered over by a lower density melt of mineral substances, the slag. Obtaining a well mixed, uniform slag is an important first step in the steel making process.
Charging lime and dolo-lime pebbles to the vessel has two inherent problems. The clean oxides of lime, CaO, and dolo-lime, CaO.MgO, are not easily wetted, and additionally, a layer or shell of calcium silicates forms on the lime pebble which retards the dissolution of CaO and MgO into the slag. The steel making process is not started until the slag is formed up because it is the CaO that acts as the primary absorbent of the impurities. The MgO is added to balance the chemistry of the slag with the refractory lining of the vessel. Without MgO in the slag it will chemically attack the refractory lining until the slag reaches chemical equilibrium with the lining. Adding MgO brings the slag into equilibrium with the refractory and extends refractory life. The MgO also has its own role in removing impurities from the metal. Thus, the time required for making up the slag, dissolving the CaO and MgO, is an essential first step in steel making, but it is also an irrecoverable loss of production time.
Al.sub.2 O.sub.3, along with CaO and MgO, is an essential constituent of any melt additive since these materials are most effective for removing impurities from the hot metal bath.
The second major problem is that lime and dolo-lime are made from limestone, calcitic and dolomitic, respectively, which are extracted from the ground by normal quarrying and/or mining processes. As a result, the limestone is produced in a range of sized and shaped pieces, from dust to boulders. The boulders can be broken down but the dust has, until this invention, been difficult to coalesce or aggregate. While the limestone is relatively hard, heating the limestone, CaCO.sub.3, is calcine it into CaO (with the CO.sub.2 driven off) yields lime which is soft and crumbles easily. Both the quarrying process and calcination step combine to produce a product with a considerable size distribution. When lime (and dolo-lime) are charged to the steel making vessel, the fine fraction and dust which are readily air-borne, are lost to the exhaust fan system of the vessel. Because the limestone quarrying and lime production steps are variable, the mix of sizes in the lime charge is a random variable. The variability of the lime charge is a serious problem for the melt shop. A charge of 140 pounds of lime per ton of hot metal to one heat may actually net 135 pounds of lime in the vessel per tone of hot metal. But the next lime charge for the next heat may be comprised of a larger portion of fines and dust with only a net of 125 pounds of lime in the vessel per tone of hot metal. The removal of silica and sulfur from the hot metal are sensitive to the portion of lime present, hence, the lime dust and fines that are lost are of no use and the variability of size distribution of the lime becomes a contributor to variability in the quality of the steel produced.
In many instances the problem is further complicated by the fact that the dolo-lime is structurally weaker than the high calcium lime. This results in a larger portion of the Dolo-lime being lost as fines and dust than for the high calcium lime.
Steel making operations, electric arc furnaces in particular, produce dust materials which are collected in control devices, usually baghouses. Scrap iron used for steelmaking often includes electric wiring components that contain solder, copper, etc., and "tin plate" which contains zinc. In the intense heat and high temperatures of the electric arc, several of the low melting point metals are vaporized: zinc, lead, and copper. The metal fume is oxidized as it condenses and escapes from the area of the arc. The electric arc furnace dust, or fume, owing to its soluble lead oxide content is a US EPA listed hazardous waste. The 1984 Amendments to RCRA (Resource Conservation and Recovery Act) have imposed an August 1988 deadline for a land ban to the disposal of electric arc furnace dust in any landfill. The complete electric arc furnace dust contains Fe.sub.2 O.sub.3, ZnO, CaO, Al.sub.2 O.sub.3, MgO, MnO, SiO.sub.2, CuO, PbO, and other substances in small amounts. The CaO and MgO are fines and dust losses from the line and dolo-lime charge. Thus, it would be advantageous to be able to recycle such dust rather than dispose of it.
Additionally, the lime producer is constrained by the portion of fines and dust that can be removed or screened from the material that is shipped to the steel melt shops. Limestone fines, usually that stone fraction which will pass a screen in the range of 3/16 to 1/2 inch, is not used as feed to the lime kilns. The limestone fines are sold, if possible, as aggregate at a low price merely to get rid of the material. Having to remove the fines and dust from the finished product is yet another problem that only erodes the net yield of the line plant. Some lime producers manufacture and sell enough hydrate (calcium hydroxide) that they can use most of the lime fines as feed to the hydrator. However most plants need to be able to ship the fines along with the pebble lime to the steel melt shops.